This invention relates to filtering screens for vibratory separation equipment such as shale shakers utilised in the oil/gas drilling industry for separating solids from the liquid phase of muds employed whilst drilling.
It is standard in the separation of solids from the liquid phase to have vibratory screens, tensioned or otherwise, adhered to a supporting sub-structure designed to fit into the cradle of a vibratory machine.
A variety of designs exist such as pretensioned wirecloths mounted and adhered to a plastic composite apertured support frame or wirecloths mounted and adhered to a perforated metal or plastic support plate or wirecloths mounted over a metal apertured support plate in itself supported by a punched metal or fabricated sub-frame.
Generally, the cloths mounted on the support media consist of two or three layers of woven wirecloth. One form consists of a filtering mesh (mesh being defined as apertures per lineal inch) over a coarse support mesh, both meshes differentially tensioned, the main functions of the support mesh being both to support the finer filtering mesh and induce a slight slapping action to prevent plugging or blinding of the filtering mesh. This is known as a two layer screen. Other constructions consist of a fine filtering mesh over an intermediate mesh, similar or slightly coarser than the filtering mesh, mounted over a much coarser support mesh. This is known as a three layer screen. Each has its own benefits and disadvantages.
The two layer screen utilising two totally disparate meshes, for example 200 mesh over a 30 mesh, provides a much superior separation efficiency with a well-defined cut point (see FIG. 1 of the accompanying drawings). On the other hand, the three layer screen because of the wirecloth combinations creates a wide range of aperture sizes across the screen which results in a much poorer separation potential and a wider range of cut points (see FIG. 2 of the accompanying drawings).
However the three layer screen has the benefit that the interaction of the three meshes can, in certain geological formations, assist in deblinding and, because of the greater mass of the cloth, give a longer life particularly at the rear of the screen which is subjected to the main initial flow of the returned mud. The user therefore has to decide whether or not to sacrifice efficiency. This invention aims to combine the benefits of both the two layer screen and the three layer screen.
According to the invention there is provided a filtering screen comprising a fine woven wirecloth filtering mesh disposed above a coarse woven wirecloth support mesh, wherein an intermediate woven wirecloth mesh is disposed between the fine woven mesh and the coarse woven mesh, the intermediate mesh extending over a proportion of the area of the screen at or towards the rear thereof, said proportion being in the range of 20% to 40% of the area of the screen.
Preferably, said proportion is substantially one third.
In a preferred embodiment, the three woven wirecloth meshes are differentially tensioned and retained in a frame, and the intermediate woven wirecloth mesh has a mesh count smaller than the fine mesh and larger than the coarse mesh.
The addition of the intermediate mesh in the rear area gives additional structural support in the area which is subjected to the greatest load, abrasion and fatigue stress whilst as the fluid flows along the screen the remaining two thirds or thereabouts will give the desired separation efficiency.